The present invention relates generally to dynamoelectric machines and, more particularly, to motors of the type that have a capacitor connected across one winding phase during operation thereof, including so-called permanent-split capacitor (or capacitor run) motors, and that are particularly adapted for energization from a conventional single phase power source.
It has long been understood in the art that it would be desirable to design single phase powered capacitor motors so that they will operate (at rated speed) in a "balanced operation" mode. For example, a paper of P. H. Trickey, "Design of Capacitor Motors for Balanced Operation", Trans. AIEE, 1932, p. 780, described a theoretical method of obtaining balanced operating conditions for single phase powered capacitor motors. This approach (as well as Trickey's paper) is further discussed in C. G. Veinott's book "Theory And Design Of Small Induction Motors", published by McGraw-Hill Book Company, Inc. in 1959 (Library of Congress Catalog Card Number: 58-14364).
According to Trickey's approach, which has been followed in the industry, a designer of such motors calculates for a specified load speed (i.e., "rated" speed), among other things, a ratio of the effective number of conductors (or turns) in the capacitor phase to the effective number of conductors (or turns) in the primary or main phase for a quadrature wound motor. This ratio is identified as the "a" ratio in Veinott's book and this same "a" ratio will be referred to and so identified herein. In theory, at least, when a capacitor motor is designed for balanced operation at rated speed, higher operating efficiencies will result; and as indicated by Veinott, during "balanced operation", a useful forward rotating magnetic field exists, but the backward rotating magnetic field is reduced to zero.
As indicated by Veinott, and confirmed by my experience, the requirement of balanced operation fixes both the winding or "a" ration, and the theoretical capacitance for the motor being designed. However, capacitors having capacitances exactly equal to this theoretical value inevitably are not available commercially. Thus, in order to actually construct a motor that would, in the real world, be capable of "balanced operation" a motor designer or engineer would have to manufacture (or have manufactured) a special capacitor having a capacitance exactly equal to the desired "theoretical capacitor".
Obviously, an approach requiring special capacitors would be so uneconomical that it could not be justified in the market place. Therefore (and again as indicated by Veinott), the practice in the art is to select a standard rating of capacitor that is nearest to the "theoretical capacitor", and that will operate near the rated or working voltage (WV) of the capacitor. Although persons skilled in the art are aware of various commercially available capacitor ratings, it is now simply noted for purposes of completeness of disclosure that capacitors are commonly available (for use, for example, at 370 WV) that have a capacitance of 4, 5, 6, 7.5, 10, 12.5, 15, etc., microfarad.
It should now be understood that, in actual practice, it has been necessary to compromise and depart from the theoretical teachings of Trickey because of capacitor availability limitations.
By way of further background, it is now pointed out that it should be possible to at least closely approach two winding balanced operation for single phase powered, permanentsplit capacitor motors by calculating a sinusoidal winding distribution (in the manner known in the art as evidenced, for example, by the above-cited book of Veinott) and "grade" the slot areas so that there would be uniform slot fill. One disadvantage of a "graded" slot approach, however, is that a core comprised of graded slot punchings is specifically optimized for a single number of poles (e.g., 2 pole, 4 pole, or 6 pole) and a fixed start to main winding shift angle. "Shift angle" means, of course, the angle by which a capacitor phase winding is shifted relative to a main winding (other than 90 electrical degrees) as taught, for example, in Linkous U.S. Pat. No. 3,82l,602.
Another problem that is frequently addressed by a motor designer involves maximizing core slot utilization (i.e., maximum filling of core slot openings with conductor material). This problem is particularly vexatious for capacitor motor applications wherein uniform slot punchings are utilized to accommodate so called "extra main" and/or tapped main windings for multiple speed motors. Such motors are disclosed, for example, in Linkous U.S. Pat. No. 3,821,602, the entire disclosure of which is incorporated herein by reference. It thus should now be understood that it would be desirable to provide single phase powered capacitor motors that can be operated in a substantially balanced mode of operation and, more particularly, that may be constructed while utilizing uniform slot punchings (so that motors of different pole numbers may be built with the same punching), and while obtaining a uniform (and preferably high) degree of slot fill, at least for multi-speed designs.
Although the above-mentioned design techniques of Trickey are for winding arrangements wherein the capacitor winding phase and main winding phase are in quadrature relationship; and improved motors of the type disclosed by Linkous include non-quadrature windings; Trickey's techniques may nonetheless be utilized in analyzing and designing non-quadrature wound motors. One way in which Trickey's techniques may be so used simply involves defining or establishing the desired non-quadrature angular relationship between the capacitor winding phase and main winding phase and then utilizing the techniques described by S.S.L. Chang in his 1956 paper ("Equivalence Theorems, Analysis, and Synthesis of Single-Phase Induction Motors with Multiple Nonquadrature Windings", Trans AIEE, "Power Apparatus and Systems", 1956, pp. 913-916) to determine an equivalent two-winding orthogonal motor, and then use Trickey's analytical approach for such equivalent motor. Thereafter, if Trickey' approach indicates a desirable modification or change in such equivalent motor; the modified equivalent motor may be used in the "synthesis" of a more optimized multiple, non-quadrature motor by following the synthesis steps described by Chang.
Utilization of all of the above-mentioned techniques has advanced the art, but experience has shown that, particularly for uniform slot multi-speed capacitor motors having more than two poles, some slots of the core will have relatively large amounts of conductor therein, while other slots of the core will have lesser amounts of conductor therein; and this is believed to be due (at least in part) to the constraints of maintaining the necessary "a" ratio involving two winding phases.
Generally speaking, when an untapped main phase winding and a capacitor phase winding are provided for a single speed motor, better slot utilization (i.e., a higher and more uniform degree of slot fill) would be otainable by distorting the winding distributions from desirable sinusoidal distribution configurations. In other words, it would be necessary to make the winding distribution less sinusoidal. This, however, would introduce the problem of establishing greater in magnitude harmonic torques which would detract from the useful output of the motor, and result in lower operating efficiency.
It is generally known that conventional three-phase motors operate in a "balanced condition", in the sense that the rotor is subjected to substantially no backward revolving field. It has also at least been proposed that a conventional Wye-connected three-phase motor (having three substantially identical winding phases A, B, C) may be energized and operated on a conventional single phase power source. This may be done by connecting the end of winding phase A to the first side of the single phase power supply; connecting the end of winding phase B to the second side of the single phase power supply; and by connecting a capacitor across the ends of winding phases B and C.
While a conventional three-phase wound motor (when connected as just described) will operate on single phase power; the above-described problems vis-a-vis multi-speed operation, and relatively uniform and optimized slot utilization of tapped or "extra-main" multi-speed motors would still remain. Moreover, such a three-phase motor would be expected to only rarely be operable in a "balanced condition"; or to only rarely produce as much full load torque (i.e., torque at rated speed when single phase powered) as it would when operated on three-phase power.
The reference that has just been made to "only rarely" is meant to indicate that, for any given conventional motor having three similar or substantially the same winding phases; there would be a capacitance value for a capacitor (in theory, at least) that would permit balanced operation of such three-phase motor from a single-phase power source. This however, brings the entire consideration of conventional three-phase motors full circle to precisely the problems (identified above, for example, in trying to implement fully Trickey's teachings) that occur because of real world capacitor limitations.
Moreover, even if a conventional three-phase motor were designed with conventional state of the art three-phase motor design techniques so as to be "matched" to an available capacitor and energized by a single-phase power source, it is believed that "balanced operation" would occur at a speed much closer to the synchronous speed (and therefore a much higher speed as compared to a more normal three-phase motor rated speed) of such motor. As will be understood, the speed point for such "balanced operation" would be such that a much lower output torque would result (as compared to the magnitude of the output torque usually available or expected if such motor were to be operated at a more usual, three-phase motor rated speed).
In view of all of the foregoing, it will now be understood that it would be desirable to provide single phase powered or energized motors having a capacitor phase (at least during running conditions) and wherein such motors would be characterized by efficiencies of a magnitude that are improved as compared to the present state of the art. It would also be desirable to provide new and improved single phase powered capacitor motors that represent a departure from the prior art in a fundamental way in order to overcome the limitations on actual attainment of balanced operation (at least because of real world capacitor limitations), and yet which may be optimized in design for different applications (e.g. different horsepower ratings; voltage ratings; pole number; and reasonable rated speeds in the vicinity of, e.g., speeds at which seventy percent of maximum torque would be available; etc.). It would also be desirable to be able to design such motors by using, to the extent possible, existing state of the art motor design techniques or procedures.
Accordingly, a general object of the present invention is to provide new and improved single phase powered motors having a capacitor phase winding (at least during running conditions) that are capable of substantially balanced operation and that therefore may have improved efficiency characteristics; and especially such motors having uniform slot punchings whereby essentially the same punching configuration may be used for 2 pole, 4 pole, 6 pole, 8 pole, etc. motors embodying the invention.
It is a more particular object of the present invention to provide fractional horsepower, single phase powered, permanent split capacitor motors that are capable of substantially balanced operation and that utilize magnetic cores having uniformly sized and spaced slot openings.
It is a still more specific object of the present invention to provide single phase powered, multi-speed capacitor motors having four or more poles wherein substantially uniform slot space factors, (i.e. degree of slot fill) are obtained in cores having uniform slots.
It is a further object of the present invention to provide new and improved single phase powered capacitor motors which are capable of substantially balanced operation and which utilize a primary phase winding, an intermediate phase winding, and a capacitor phase winding.
More generally stated, other objects are to provide new and improved single phase powered capacitor motors which are capable of high speed operation that more closely approaches balanced operation than heretofore, and yet which are not subject to previously known restrictions vis-a-vis calculated theoretical capacitors.